Diocese in Lower Austria, derives its name and origin from Fanum Sancti Hippolyti, a monastery founded there in the ninth century and dedicated to St. Hippolytus. The origin of this monastery is obscure. Some think that monks from Lake Tegernsee in Bavaria founded a Benedictine abbey on the Traisen in 791, when Charlemagne united a part of the territory of the Avars with his empire, and Passau took this district as a mission field. In the ninth century Sankt Pölten was the eastern limit of Christian civilization, the only monastery east of the Enns. It is said that the monastery was transferred to secular canons in 985, and in 1080 the great reformer Altmann of Passau replaced these by Reformed Augustinian Canons. The first provost was Engelbert. The bishops of Passau attached much importance to the spiritual and material improvement of this important support of their power in the east. Hefele in his "Konziliengeschichte" (VI, pt. II, 230-2) gives the decisions of the synod that Bishop Gottfried of Passau held at Sankt Pölten in 1284. These were of importance: if a priest celebrates solemnly the wedding of his son or his daughter, he is to be suspended; the secular clergy, pastors, vicars, and chaplains must confess their more serious sins to the dean, the latter to the bishop or archdeacon; everyone may confess less serious sins and negligences to whom he will. Annates are mentioned even at this early date; "the first year of the episcopal collation of vacant churches is used for the church at Passau". Another synod was held at Sankt Pölten ten years later.

Soon after this (1306) the city came very near destruction. As in other places stories were current of sacrilegious acts of Jews, especially of pierced and bleeding Hosts. These tales led to the founding of churches of the Sacred Blood; and at Sankt Pölten, as elsewhere, the Jews were robbed and murdered. Only the intercession of Bishop Wernhart prevailed upon King Albert I not to destroy the city. When the Reformation began, the monastery of Augustinian Canons was not strong enough to withstand it; in 1565 there were only three canons. Aid, however, was given by Klesl (q.v.) and the Jesuits, through whose efforts many citizens were converted. Part of one of Klesl's sermons is preserved in the city archives: "Behold, for a thousand years the pictures of your forefathers holding rosaries in their hands have stood in this church". In 1706 the first settlement of the Institute of Mary (q.v.) was made at Sankt Pölten, whence they had been called from Munich by the vice-president of the Government of Lower Austria, Jakob Freiherr von Kriechbaum. At the same time Carmelite nuns settled there. They were later suppressed by the Emperor Joseph II, and the same fate befell the monastery of Augustinian canons. The fifty-ninth and last provost was Ildefons Schmidtbauer. The emperor took the monastery for the episcopal residence and the monastery church for the cathedral. As the Diocese of Wiener-Neustadt reached almost to the capital, Vienna, Joseph II united its territory with the Archdiocese of Vienna, and transferred its bishop to Sankt Pölten. A new diocese was established at Linz and both bishops were made suffragans of the Archbishop of Vienna.

Since 1785 Sankt Pölten has had thirteen bishops, each episcopate averaging less than ten years. A popular tradition relating that the last provost had predicted that no bishop would reign over ten years was, however, disproved by the tenth bishop, Feigerle, who reigned eleven years. Some of the bishops have been very distinguished: Sigismund, Count Hohenwart, who was tutor of the Emperor Francis and the Archduke Charles and became Prince Archbishop of Vienna; the court preachers Jakob Frint, Michael Wagner, and Ignaz Feigerle; above all Joseph Fessler, the learned professor, skilful diplomatist, and secretary of the Vatican Council (d. 1872). In 1836 Johann Leonhard resigned the bishopric. At present the diocese has two seminaries for boys, which train candidates for the priesthood. Fessler united one of these seminaries with the seminary at the Benedictine Abbey of Seitenstetten; the other was established at Melk by the present Bishop Johann Rössler. In 1908 Rössler held the first diocesan synod of the independent Bishopric of Sankt Pölten; the important constitutions and acts of this synod have been printed. The Diocese of Sankt Pölten contains 620,000 Catholics; 479 secular priests; 505 members of male orders in 16 houses; and 874 members of female orders in 94 branch houses.